1. Field of Invention
The present invention relates to a type of laser repair. More particularly, the present invention relates to a laser repair operation of a silicon wafer before conducting a bump-forming process.
2. Description of Related Art
Integrated circuits are complicated electronic products whose manufacturing involves a series of major activities including integrated circuit design, wafer fabrication, wafer testing and wafer packaging. In general, manufactured integrated circuits must undergo a series of tests before shipping just to ensure a high product quality. The results provided by these tests are essential also for repairing any malfunctional integrated circuits. As usual, natural yield of integrated circuit is relatively low. Hence, on discovering some defects in the circuit of a silicon chip, a laser repair operation is often conducted. In the repair process, a number of fuses are generally burnt by a laser so that specially designed redundant bit lines can replace the defective bit lines. Ultimately, product yield of the wafer may increase considerably.
FIGS. 1 through 4 are schematic cross-sectional views showing the progression of steps in a conventional laser repair operation. First, a silicon wafer is provided. The wafer has a plurality of silicon chips 100. In FIG. 1, only a single silicon chip 100 is shown. The silicon chip 100 includes a plurality of bonding pads 102, a plurality of testing pads 104, a plurality of fuses 106 and a passivation layer 108. The passivation layer 108 protects the silicon chip 100 and includes openings that expose the bonding pads 102 and the testing pads 104.
As shown in FIG. 2, a probe 110 is used to early out a testing operation. A probe mark 112 is formed on the testing pad 104. When any defects are found lathe circuit a laser repair operation is next carried out. In a laser repair, a laser beam aims at a fuse 106, burning a portion of the passivation layer 108 and melting open the protective fuse 106 to form a fused section 114. Specially designed redundant bit lines then replace the defective bit lines. Through the laser repair operation, the yield of silicon chips on a wafer is greatly boosted.
As shown in FIG. 3, a bump-forming process is carried out after the laser repair operation. The bump-forming process mainly includes under ball metallurgical (UBM) layer fabrication and bump production. Since the fabrication of the UBM layer often requires etching to form the pattern, a second passivation layer 116 is formed over the silicon chip 100 to prevent unwanted etching of the testing pad 104 and the burnt section 114. To be useful as a protective layer, the second passivation layer 116 needs also to undergo a photolithographic and etching process to form an opening 117 that exposes the bonding pads 102.
As shown in FIG. 4, a conductive layer is formed over the chip 100. Photolithographic and etching processes are conducted to form a bottom metallic layer 118 over the bonding pad 102. Finally, a bump 120 is formed over the bottom metallic layer 118, thereby completing the process of conducting a laser repair and fabricating bumps on a wafer.
Because a bump-forming process is carried out after a laser repair and an etching step is used to form the bottom metallic layer in the bump-forming process, a second passivation layer is required to protect the laser burnt fuse area. In addition, a masking step is required to form an opening that exposes the bonding pad in the second passivation layer so that a bump can be formed on the bonding pad. In general, the coating of a second passivation layer and the forming of an opening in the second passivation layer complicate the fabrication process and increase production cost.
Accordingly, one object of the present invention is to provide a laser repair operation. The laser repair operation includes fabricating a bottom metallic layer on bonding pads and testing pads, conducting a testing operation by probing the bottom metallic layer on the testing pads, and finally performing a laser repair. Since etching that might damage the exposed fuse is no longer conducted after a laser repair, forming a second passivation for protecting the broken fuses is unnecessary.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of conducting a laser repair operation. A silicon wafer having a plurality of chips thereon is provided. Each chip has a plurality of bonding pads, a plurality of testing pads, a plurality of fuses and a passivation layer for protecting the chip. The passivation layer exposes the bonding pads and the testing pads. A bump-forming process is conducted to form a bottom metallic layer and a bump sequentially over each bonding pad. A bottom metallic layer is formed over each testing pad. The bumps are formed, for example, by electroplating or printing. Thereafter, testing is carried out by probing various bottom metallic layers above the testing pads. Finally, a laser repair is conducted.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.